1) Field of the Invention
The present invention relates to a process for producing a heat-sensitive recording material. More particularly, the present invention relates to a process for producing a satisfactory whiteness and an enhanced heat-sensitivity.
2) Description of the Related Arts
A heat-sensitive recording system is advantageous in that colored images can be recorded on a recording material by only a heating operation and the recording apparatus is relatively simple and compact. Accordingly, this heat-sensitive recording system is widely utilized for various information-recording systems.
Particularly, due to recent extensive developments in facsimile systems in which a heat-sensitive recording printer is utilized, it has become possible to provide a high recording speed impossible to obtain from a conventional recording system.
Accordingly, there is a strong demand for the provision of a heat-sensitive recording material having an enhanced record sensitivity and usable for a high speed recording machine.
Many attempts have been made to meet this demand, and usually, the heat-sensitive recording material comprises a substrate composed of a paper sheet, plastic film or synthetic paper sheet, and at least one heat-sensitive recording layer comprising, as a principal component, a heat-sensitive color-forming material and a resinous binder and formed on the substrate.
It is known that the recording sensitivity of the heat-sensitive recording material can be enhanced by:
1) adding a heat-fusible substance having a low melting point to the heat-sensitive recording layer, PA1 2) increasing the surface smoothness of the heat-sensitive recording layer, and PA1 3) increasing the content of the color-forming material in the heat-sensitive recording layer. PA1 (1) triacrylmethane compounds, for example, 3,3'-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide; PA1 (2) diphenylmethane compounds, for example, 4,4'-bis-dimethylaminobenzhydrinbenzylether, N-2,4,5-trichlorophenylleucoauramine, and N-2,4,5-trichlorophenylleucoauramine; and PA1 (3) xanthene compounds, for example, rhodamine B-anilinolactam, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-7-butylaminofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-ethyltriamino-6-methyl-7-anilinofluoran, 3-cyclohexyl-methylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-(.beta.-ethoxyethyl)aminofluoran, 3-diethylamino-6-chloro-7-(.gamma.-chloropropyl)aminofluoran, 3-ethyl-isoamylamino-6-methyl-7-anilinofluoran and 3-dibutylamino-7-chloroanilinofluoran. PA1 (1) phenol compounds, for example, p-octylphenol, p-tert-butylphenol, p-phenylphenol, 1,1-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)pentane, 1,1-bis(p-hydroxyphenyl)hexane, 2,2-bis(p-hydroxyphenyl)hexane, 1,1-bis(p-hydroxyphenyl)-2-ethylhexane, 2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane and dihydroxydiphenylether, and PA1 (2) aromatic carboxylic acid compounds, for example, p-hydroxybenzoic acid, butyl p-hydroxybenzoate, 3,5-di-tert-butyl salicylic acid, 3,5-di-.alpha.-methylbenzylsalicylic acid and multivalent metal salts of the above-mentioned acid compounds.
Nevertheless, the above-mentioned approaches are disadvantageous in that an undesirable adhesion of the heat-fusible component in the heat-sensitive recording layer to a thermal head in the printer occurs, and this adhesion causes a formation of unclear images on the recorded material.
Many attempts have been made to remove the above-mentioned disadvantages, and it has been found that a fine pulverization of the heat-sensitive color-forming material particles effectively enhances the recording sensitivity of the heat-sensitive recording layer without increasing the content of the color-forming material in the recording layer.
For example, Japanese Unexamined Patent Publication Nos. 58-69089 and 58-76293 disclose a process for finely pulverizing the heat-sensitive color-forming material.
It is commonly believed that, in a heat-sensitive recording layer having a high recording sensitivity, the heat-sensitive color-forming material must be in the form of fine particles having an average particle size of 0.7 .mu.m or less, but it is very difficult to attain the fine pulverization of the heat-sensitive color-forming material as long as the pulverization is carried out by using a dispersing medium consisting of solid grains having a diameter of 1.0 mm or more.